Properties of action potentials from insect motor nerve fibres.

Extracellular recording from small nerve branches provides a method for recording from several nerve fibres simultaneously. This is extremely desirable in studying the organization of a neuronal system. Furthermore, extracellular records are insensitive to small movements, are stable over long periods, and can easily be repeated with subsequent experimental animals. They also involve relatively little dissection, and hence reduce the risk of injury to nerve fibres. We have derived various theoretical predictions for records taken from small nerve trunks using different electrode configurations (see Pearson, 1969). The present paper illustrates some of these theoretical results and their application to experimental work. The preparations we have used are a levator motor nerve from the metathoracic segment of the cockroach and the tergal nerve to the third abdominal segment of the locust. The cockroach nerve is not ideal for this particular study, but is of interest in our work on the functional organization of motor units (Pearson & Bergman, 1969; Pearson & lies, 1970) and on the patterns of neural activity during behavioural sequences. Use of a 'typical', rather than a specially selected nerve, illustrates the advantages and disadvantages of the various recording methods. The locust nerve, on the other hand, was selected because of its long length. The following properties of action potentials from different nerve fibres were measured in these preparations: monophasic and triphasic amplitudes, conduction velocity and duration. Triphasic or monophasic amplitudes were used to predict fibre diameter, as checked from histological sections. Conduction velocity was less useful since there were systematic deviations from the expected square root relation between velocity and diameter. The systematic effects of fibre diameter on the properties of nerve action potentials should be of general physiological interest.